The correct answer is - rainfall.
The rainfall is crucial and the most important factor in the creation of the soil. The rainfall has such big influence because of few things:
- weathering
The water from the rainfall manages to weather the rocks, making small pieces of sandy particles from them.
- plant life
The plant life needs water to exist, and the rainfall provides that. Further, the plants that die are the biggest factor that contributes to the soil formation and fertilization.
- animal life
Once there's rainfall and plants, animals can exist too, and the animals are very important in both the fertilization of the soil, and fertilization of the plants that also fertilize the soil.
- decomposition
The dead plants and animals much quicker and much more efficiently when there's bigger humidity and water, thus the rainfall is essential for that.